A major challenge associated with design of a microwave oven is keeping the electromagnetic energy (EMI) contained within the oven cavity where the food is prepared without leaking out into exterior spaces surrounding the oven cavity through openings, gaps in the seal, and through the materials that form the boundaries of the oven cavity.
The most difficult area of the oven cavity to seal off is the, always present, gap between oven door and cavity. To stop energy from leaking through this gap a so called ‘choke’ is commonly used. This choke is basically a folded section quarter wave transformer that reflects and dampens any EMI trapped inside the choke.
The physical dimensions of the choke are dictated by the frequency of the radiation to be contained. Ideally the depth of the choke is equal to a quarter of the wavelength (214) of the radiation to be contained. For example, for a frequency of 2.45 Gigahertz (GHz), the ideal choke depth is 30.6 millimeters (mm). In many implementations of household microwave ovens, chokes are generally vertically oriented inside a plane of a door to the oven cavity, which helps minimize a thickness of the door. In addition, some household microwave ovens also use chokes having a depth of less than (λ/4) to further save space.
However, to achieve high levels of EMI shielding required by recent commercial airline standards, the choke for aircraft microwaves cannot have a depth of less than (λ/4), which results in the chokes for aircraft microwave ovens being larger than household microwave ovens. In addition, the bezel for the aircraft microwave oven doors that forms a counter-shape for the larger, vertically-oriented choke is significantly bigger than bezels for household microwave ovens with smaller, thinner chokes, which severely restricts the size of a door opening for the microwave oven cavity.